This invention relates to the reaction of organochlorosilanes with methanol in the presence of a tetraorganophosphonium chloride catalyst to produce organosiloxanes, methylchloride and water.
At present the primary commercial method for preparing methylsiloxanes is by hydrolysis of methylchlorosilanes to give methylsiloxanes and hydrogen chloride. Recovered hydrogen chloride is reacted with methanol to produce methylchloride which in turn is reacted with silicon metal to form the methylchlorosilanes. The chlorine cycle constitutes three steps: (1) hydrolysis to give hydrogen chloride, (2) reaction of hydrogen chloride with methanol to give methylchloride, and (3) reaction of methylchloride with silicon metal. It is highly desirable to reduce this to a two step cycle by combining steps (1) and (2) so that chlorosilane is reacted with methanol to give siloxane and methylchloride according to the equation EQU Me.sub.n SiCl.sub.4-n +(4-n)MeOH.fwdarw.(4-n)MeCl+(4-n)/2H.sub.2 O+Me.sub.n SiO.sub.(4-n)/2
where n can be 2 or 3 and Me represents the methyl radical.
Reactions of chlorosilanes with methanol to give organosiloxanes and methylchloride according to the above equation are not new. U.S. Pat. No. 2,556,897 discloses a liquid phase reaction of dimethyldichlorosilane with methanol. No catalyst is suggested. A viscous siloxane product was obtained. U.S. Pat. No. 2,741,630 describes the same reacion at a temperature of 175.degree. C. in the presence of ZnCl.sub.2 on silica gel. U.S. Pat. No. 3,803,195 shows the reaction of methanol with Me.sub.n SiCl.sub.4-n by a countercurrent flow method in which the column is packed with an essentially inert and acid resistant packing material. The patent teaches col. 4, lines 6-12, that catalytic agents such as Lewis acids and cation exchange resins in the H-form may be used with the packing materials, but the use of such materials is not desirable and should be avoided since these materials tend to promote the cleavage of silicon-carbon bonds.
U.S. Pat. No. 4,108,882 by the same inventor as the instant application describes the vapor phase reaction of methanol and methylchlorosilanes in the presence of quaternary ammonium chloride salt catalyst. The method results in improved yields of methylsiloxanes and methylchloride without significant cleavage of silicon-carbon bonds. In addition, the method gave only low yields of dimethylether as a contaminant in the methylchloride.